Duchenne muscular dystrophy (DMD) is an X-linked, inherited disease affecting 1 in 3500 live newborn males. The disease results in progressive muscle weakness, endomysial inflammation, and fibrotic scarring. Muscle fibrosis is deleterious in multiple ways: It reduces normal transit of endomysial nutrients through connective tissue barriers, reduces the blood flow and robs muscle of vascular derived nutritional constituents, and functionally contributes to early loss of ambulation through limb contractures. Over time, treatment challenges multiply as a result of marked fibrosis in muscle. Successful treatment of DMD mandates reduction of muscle fibrosis potentially achieved with microRNA-29 (miR-29). miR-29 has been used successfully in experimental paradigms to reduce pulmonary, liver, and skeletal muscle fibrosis, and it is the intent of this proposal to demonstrate through proof-of-principle that muscle fibrosis can be reduced using miR-29 delivery by AAV. I hypothesize that over-expression of microRNA-29 using adeno-associated virus (AAV) delivery to muscle will reduce fibrosis in mdx and mdx:utrophin heterozygous (mdx:utrn+/-) mice. This hypothesis will be tested using a sequential paradigm through the following three aims: 1) Determine the expression level of miR-29a, miR-29b, and miR-29c in muscle biopsies of DMD patients and mdx:utrn+/- mice, and correlate these findings with existing levels of fibrosis; 2) Deliver AAV.miR-29 to mdx and mdx:utrn+/- skeletal muscle and study the expression of the microRNA and its effect on endomysial connective tissue; 3) Test the therapeutic efficacy of combinational therapy using AAV-delivered miR-29 with AAV-delivered micro-dystrophin. The overall strategy can be summarized as follows: the miR-29 isoform(s) that is most correlative with muscle fibrosis will be identified, overexpressed in DMD mouse models via AAV to determine its potential therapeutic efficacy on fibrosis reduction, and then delivered as combinational therapy with micro-dystrophin gene replacement. Outcome measures will include determining levels of fibrosis, muscle force generation and protection against eccentric contraction. Successful demonstration in the a severe murine model of the disease (mdx:utrn+/- mouse) paves the way for further testing in canine dystrophy with the potential to take to clinical trials. The project outlined here is feasible and stepwise in its approach, addressing an important translational question with high impact related to the potential treatment of muscular dystrophy.